I've done everything and I can't get to the 198 mg/L value for my GGA. Here is a list of what I've done:

I'm using polyseed.

Hydrate/ aerate seed for 1h, 2h, overnight. Overnight one add few mL of GGA as food. For every test, the following volumes of seed have been tested, 2, 3, 4 and 5 mL. For some test I've tried even 10 mL.

-Different lot number of seed, same results

-Try hydratation of seed with 35 C of dilution water.

-Try 1, 2 or 3 capsule of polyseed.

-Try 2% and 1% GGA dilution same results

-Used a different source water, purchased Steam distilled water from Nursery. This water had fluoride and minerals added. From this test got values a bit higher for the 4mL of seed, 155, 158, 157 mg/L and 5 mL of seed, 157, 157 and 158mg/L for 2% GGA solution. Still not much of an improvement.

Blanks are below 0.2mg/L, and the seed depletion/mL are between 0.21 to 0.35 mg/L per mL of seed.

I've ask to colleagues to analyze source water for heavy metals, all were below MDL (Cu lower then 2 ppb), and for TOC lower then 0.2ppm.

Could the nutrient solutions played a role in affecting the GGA results? Any suggestions are welcome...

Your seed depletion of 0.21 - 0.35 is very low. The seed depletion should be between 0.6 - 1.0. I use a seed from NCL with very good results. You also might try using settled primary effluent for seed.

Polyseed has no nitrifying bacteria by design, so when you think you are doing a BOD, you are actually doing a CBOD since the nitrogen-containing materials in the sample (the glutamic acid in the case of GGA) are not oxidized to nitrate. So you would expect to get low GGA results with Polyseed.

The commercial seed sold by NCL (mentioned in the previous post) is made by BioSystems which, according to the manufacturer, also has no nitrifiers. However, my experience in evaluating several labs using BioSystems seed is that it usually performs better than Polyseed.

If your lab is at a wastewater treatment plant, the best seed is close at hand, and free. Primary effluent (such as runoff from a clarifier) should have a good population of hungry, healthy bacteria. The supernatant liquid from settled sludge is sometimes a good source of bacteria. Raw influent sometimes works, but is often very inconsistent because of natural and human activities that influence the bacteria. If your wastestream includes industrial waste, watch for the possibility of toxicity that might weaken or kill the bacteria.

My best advice to you, if you are at a WWTP, is to try taking your seed from your own wastestream and use a commercial seed as a last resort. If you are at a commerical lab, try using a seed from a local WWTP that is doing well on its own GGAs. If that works, you may want to use that seed as a permanent fix...I know may commericial labs that have had success doing that, keeping the seed sample in their incubator until a control chart shows it is starting to weaken.

We use the Biosystems seed material and it seems to work with more consistency than the polyseed. It is a higher demand seed material, thus more media is left with the bacteria, and they seem to recover from dehydration better. You can also use settled fresh sewage if there are not an excess of nitriers. We get near the 198 value for both BOD and CBOD.

I would surely get a new lot of GGA standard, maybe from another manufacturer, to ensure the standard is not in error. You can try new nutrient solutions, or nutrients from Hach already mixed, for comparison. The cost is not much. Find a lab that does not have a problem with BOD standards and have them use some of your source water to see if that is the cause.

Using seed material that contains nitrifiers is not the answer as ammonia from the sample and ammonia from the dilution water will both contribute to the result. This is why the BOD test has such notorious reputation as a bad procedure. Different dilutions yeild big diferences making comparison among labs difficult.

Are you making your GGA yourself? If so, are you desiccating the glucose and glutamic acid prior to weighing out? If you don't desiccate to remove excess moisture you will actually be making a standard at a lower concentration.

Perry, we are doing BOD but I guess since we are using B.O.D. Seed (Or polyseed) and they have no nitrifing bacterias, it like doing CBOD for the GGA. Also since we d'ont add nitrification inhibitor to our sample we have to report BOD. In addition we analysed our PT samples and we were not even near the CBOD values.